Antennas with omnidirectional radiation patterns are desired in radio frequency communications systems such as cellular telephones used in automobiles and the like. Typically, vehicle-mounted antennas for cellular communications consist of a short wire or "whip" antenna extending approximately 3.048 meters from the vehicle. this type of cellular antenna is undesirable for a variety of reasons including a greater risk of breakage and interference with other structures around the vehicle (e.g. dynamic mechanisms within a car wash).
Attempts have been made in the prior art to implement cellular telephone antennas utilizing a surface mounted antenna with a low profile. For example, U.S. Pat. No. 5,041,838 describes a planar antenna employing a dielectric substrate with conductive coatings on two major surfaces, one surface is connected to an outer ground shield of a coaxial cable and the other surface is connected to an inner conductor. A plurality of electrically conductive shunts are disposed along a radius of the antenna and interconnect the two major surfaces. Another shunt, not on the same radial line as the plurality of shunts, is also disposed to interconnect the two major surfaces and to match the electrical characteristics of the resulting antenna to those of the transceiver. The location and number of the shunts are experimentally varied in order to adjust the antenna's impedance to resonate within the desired frequency band.
In addition many of the wireless and cellular applications discussed above exhibit simultaneous requirements for GPS resources. Examples of such applications may be in determining transportational logistics and vehicle tracking. For instance, it is often the case that a motorist is stranded on a roadway without specific knowledge of his or her actual location or of how to direct a third party who wishes to locate the stranded motorist. By combining a cellular antenna with access to GPS resources the stranded motorist could simply depress a button linked with the cellular phone which would result in a signal being sent over the cellular band indicating the precise location of the stranded vehicle to a cellular base station as determined using the associated GPS resources. Such a system also becomes useful in military applications where the tracking of military vehicles in order to yield a set of precise, real time coordinates of each of a number of various vehicles is particularly useful when strategic positioning must be dynamic. Currently, as evidenced by the prior art, existing cellular and GPS antennas represent distinct physical structures mounted individually and in different locations on the vehicle. However, due to functional as well as aesthetic considerations, it is desirable to be able to eliminate the dual mounting of two separate units for GPS and cellular communication by having a single module for accomplishing both functions.
It is an object of the present invention to combine a low profile antenna having an omnidirectional radiation pattern for use in microwave communications systems such as cellular telephone systems with a GPS antenna in the same compact physical structure whereby the performance of both the low profile antenna and the GPS antenna and preamplifier will not be degraded by interference between the two antennas.
It is a further object of the present invention to provide a low profile antenna having an omnidirectional radiation pattern for use in microwave communications systems such as cellular telephone systems.